We use the #1 brand (made in the USA) of 30 mil magnets specially-made for promotional vehicle use. It will stick to any flat vehicle surface made of metal. Please be aware that it will not work on fiberglass/plastic doors or areas of the vehicle which have had extensive body damage (corrected using Bondo or putty).
Magnetic car signs work best on flat surfaces. As long as it is not placed upon a curved surface and you follow the magnetic care instructions, you should not have to worry about the magnet falling off.